Transistors of integrated circuit devices have uncertainty in a number of parameters. For example, process variations may lead to transistors formed in a die of a silicon wafer or across the wafer to have different effect lengths (Leff) of the gate region, different threshold voltages (VT) for turning on the transistor, or different thicknesses of the gate oxide (tOX). For example, a device may be fast if it has a short effective length, a low threshold voltage, or a thin gate oxide. Conversely, a device may be slow if it has a long effective length, a high threshold voltage, or a thick gate oxide. In modern complementary metal oxide semiconductor (CMOS) process, a CMOS pair comprising n-type metal oxide semiconductor (NMOS) transistor and a p-type metal oxide semiconductor (PMOS) may have different “strengths.” The combination of these or other parameters may affect the characteristics of each of the devices of the CMOS pair. Because some parameters are independent for NMOS and PMOS devices, one device of the CMOS pair may have one characteristic, while the other device may have another characteristic.
The process corners for CMOS devices show a characteristic of the transistor, such as slow to fast, with an NMOS transistor plotted on one axis and a PMOS transistor plotted the other axis as shown for example in FIG. 1. The four corners are defined by a slow-slow (SS) corner, where both the PMOS transistor and the NMOS transistors are slow, a fast-fast (FF) corner, where both the PMOS transistor and the NMOS transistors are fast, a slow-fast (SF) corner, where the PMOS transistor is slow and the NMOS transistors is fast, and a fast-slow (FS) corner, where the PMOS transistor is fast and the NMOS transistors is slow. A CMOS pair having typical PMOS and NMOS transistors is designated as a TT CMOS pair which is located somewhere in the middle of the four corners on the grid. Digital and analog circuit designers are facing more and more challenges to optimize their circuits and meet performance goal across all process corners.